There is currently one main obstacle to overcome in the realization of a spectral CT scanner based on photon counting detectors: the count rate limitations of state-of-the-art detector systems to about 5-10 million counts per second and pixel. Conventional CT systems are optimized for short scanning times and are therefore operated at very high photon flux rates of about 1 billion counts per second and pixel. Thus, there is a discrepancy between the count rate of available detectors and the count rate required for CT systems.
This count rate problem is a severe burden for the application of photon counting detectors in CT. The count rate limitation in CT is mainly for X-rays at the object (e.g. patient) periphery and outside the object (called peripheral rays). The majority of rays passing the object are sufficiently attenuated to not overload a counting detector. Although the measure of rays outside the object are not important for the imaging task, the overload of detector readings in the peripheral areas creates severe image artefacts and cannot be accepted for clinical imaging. Both known concepts have severe disadvantages:
European patent application No. 08171898.3 (PH011734EP1), which has not yet been published, describes an X-ray examination device and a corresponding method according to which the source current of the X-ray source is modulated between at least two different source currents to obtain at least two detection data sets for at least two different X-ray fluxes, wherein the lowest X-ray flux is low enough to avoid overloading of the X-ray detector in the direct X-ray beam. An X-ray image is reconstructed from said at least two detection data sets, wherein the pixel values of the pixels of said X-ray image are reconstructed taking into account whether or not the higher X-ray flux resulted in an overloading of the X-ray detector at the respective detector cells. In this way, however, the angular sampling of both acquisition types (low flux, high flux) is reduced.